3.4.4 Lexical Scope

The rules that we have just been describing are the details of how
Scheme implements “lexical scoping”. This subsection takes a brief
diversion to explain what lexical scope means in general and to present
an example of non-lexical scoping.

“Lexical scope” in general is the idea that

an identifier at a particular place in a program always refers to the
same variable location — where “always” means “every time that the
containing expression is executed”, and that

the variable location to which it refers can be determined by static
examination of the source code context in which that identifier appears,
without having to consider the flow of execution through the program as
a whole.

In practice, lexical scoping is the norm for most programming languages,
and probably corresponds to what you would intuitively consider to be
“normal”. You may even be wondering how the situation could possibly
— and usefully — be otherwise. To demonstrate that another kind of
scoping is possible, therefore, and to compare it against lexical
scoping, the following subsection presents an example of non-lexical
scoping and examines in detail how its behavior differs from the
corresponding lexically scoped code.